GB2037731A - Reactor for the anaerobic decomposition of organic sludge and for the production of methane - Google Patents

Abstract

The reactor is a horizontal cylindrical vessel (1) in which an agitator (13) rotates around a horizontal central shaft (2). The vessel has an inlet (6) for sludge to be digested at one end, an outlet (7) for digested sludge at the opposite end, and a discharge pipe (9) in its upper part for methane. Partitions (10), attached to the agitator to rotate therewith, divide the vessel longitudinally into chambers, the last of which forms a settling chamber (14) which contains return pipe means (17) for returning part of the solid matter settled in the settling chamber to one of the chambers at the inlet end of the vessel. The pipe (17) extends at an angle to the shaft (2) and rotates with it. The return means includes a scraper (18) for pushing settled solid matter into the return pipe. <IMAGE>

Description

SPECIFICATION A method and a reactor for the anaerobic digestion of organic sludge and for the production of methane gas.

This invention concerns the anaerobic digestion of organic sludge in order to produce and recover methane gas. In particular, the invention is a reactor comprising an essentially horizontal cylidrical vesel housing an agitator rotating around a horizontal axis, an inlet at one end of said vessel for sludge to be digested, an outlet at the opposite end for removing the digested sludge from the vessel, and a pipe for recovery of the methane gas in the upper portion of said vessel.

This invention also comprises a method for the anaerobic digestion of organic sludge and a method for the production of methane, in which the sludge to be digested is fed into a closed reactor in which the sludge is stirred and possibly heated, and from which both digested sludge and methane gas are discharged.

The purpose of the invention under consideratin is to increase the methane yield during anaerobic digestion of organic matter. Digestion is a means of breaking organic matter down, primarily using methane bacteria, so that digestion gas composed primarily of methane and carbon dioxide is producted. This digestion gas is a valuable fuel.

Digestion is a common method for treating the sludge produced at municipal waste water treatment plants. It is foremost a means of making the sludge more hygenic and improving its drying properties; it also yields enough digestion gas to produce at least the heat needed for maintaining the process temperature.

The difficulty has in general been to obtain a process and reactor economical in terms of energy consumption when small units are involved. However, it has been recognized that even a small increase in the digester gas yield would make the process economical, particularly as the price of energy is constantly on the rise.

Several methods for increasing the production of digester gas in the anaerobic digestion of organic sludge are already known. Attempts have been made to increase production e.g. by raising the temperature of the process, stirring the sludge effectively in the reactor, dividing the process into sections suitable for different microbe groups, adjusting the acidity, increasing the various organic or inorganic nutrients and by eliminating the detrimental effects of certain heavy metals or sulphide ions.

It is also known that methane bacteria are very sensitive to oxygen, primarily because oxygen is actually toxic to them, and also because methane bacteria can function only under reducing conditions. The sludge can obtain oxygen from either the atmosphere or from chemical reactions. In practice small amounts of oxygen always enter the digestion reactor when the sludge is fed into the vessel in the form of air bubbles, when the sludge comes into contact with the air in the feed wells or sludge channels, when air is sucked through the pump seals, etc. In general, technical sealing methods have been proposed for prvention of these detrimental effects.The use of reduction to remove oxygen from sludge is generally recognized in microbiological techniques when there is a desire to further the growth of anaerobic bacteria under experimental conditions, as Mylroie and Hungate have shown in Can.J.Microbiology, Vol.1, pp.55-64(1954).

There are also numerous substrate formulations for microbiological cultivation.lt has now been found unexpectedly that the detrimental effect of oxygen leaks can be combated more effectively by adding ferrous sulphate to organic sludge which is to be digested. The ferrous sulphate either binds directly with the oxygen that has entered the reactor, or lowers the redox potential in the reaction space, thereby favouring the type of chemical or biological reactions by means of which the oxygen can be bound before it is able to hinder the action of the methane bateria.

The production of digester gas was raised c. 50-60% by using ferrous sulphate in comparison with reactors operating without this reducing agent. The results are presented in the table below.

TABLE Gas production in experimental reactors at various load levels Load Gas yield l/lr.d GVSll,.d comparison FeSO4-l FeSO4-ll 3.6 1.2 1.5 1.4 4.7 1.5 2.0 1.7 6.1 1.9 2.5 2.4 6.8/1 week 1.8 2.7 2.7 6.8/2 weeks 1.9 3.0 2.8 6.8/3 weeks 1.7 2.7 2.8 VS = Volatile Solids 1r = reactor volume in litres I = gas yield in litres d = day FeSO4-l = smaller amount of reducing agent FeSO4-ll = larger amount of reducing agent The above table indicates that both of the reactors to which ferrous sulphate was added produced substantially more gas than the control, to which ferrous sulphate was not added. In contrast, there was no difference between the larger and the small amounts of reducing agent. This was because both amounts were unnecessarily large.

In waste water treatment ferrous sulphate is commonly added to remove phosphorus in the so-called simultaneous precipitation method, and for the elimination of odour from sewage, precipitation basins and the like (where ferrous sulphate binds with the hydrogen sulphide) and for elimination of sulphide inhibition.

In all these methods use has been made of the tendency of iron to react chemically with the substance to be removed. Use was not made of the reducing property of the iron in ferrous sulphate. Instead, ferrous sulphate was used in the above cases only because it is inexpensive in comparison with other iron salts. The use of ferrous sulphate in the method employed in the invention under consideration is not, however, due to the low price alone. The main reason is that ferrous sulphate effects a considerable increase in the yield of digester gas in comparision with previous methods.

Since methane bacteria reproduce slowly, it has been suggested that part of the bacteria mass be returned to the beginning of the process as a seed in orderto increase the production of gas. However, the toxic effect of oxygen is at a maximum at the input end of the reactor. The method employed in the invention under consideration does, however, make elimination of this toxic effect possible, and thus the methane bacteria returned to the input end of the reactor are able to continue their activity without interruption. The seed is best recovered from settled sludge, which contains the most solid matter. Return is usually accomplished with a pump, although in this case there is a risk that oxygen will enter the sludge via the shaft or pipe seals.

The purpose of the invention under consideration is in fact to create a reactor in which these detrimental effects are overcome.

The ferrous sulphate is added just before addition of the seed. This is best performed in the pre-treatment section where the sludge can also be heated to the reaction temperature. Construction of a separate pre-treatment or pre-heating section will, however, raise the price of the reactor. Moreover, pumps and pipes must be mounted between the various sections, and these cause technical problems and the risk of leaks.

At present the reactor models used for the anaerobic digestion of organic sludge are in general vertical cylinders which are difficult to divide into sections. They contain numerous seals for the agitators, pump shafts and other auxiliary equipment, and these create the risk of leaks. This means that oxygen can enter and hinder the action of the methane bacteria. Methane gas can also escape, and this means there is a risk of explosion. Moreover, the hydraulic pressure in vertical reactors is high, which is unfavourablefor methane bacteria. Stirring is also uneven, and demands a great deal of energy or complex and expensive pumps.

Horizontal reactors have been known before (Siert, Fr., Splitteberger, A. and Holdhdfer, H., Handbuch der Lebens-mittelchemi, Wasser und Luft, Erster Teil, Springer-Verlag, pp. 327 (1939) and DE Offenlegungsschrift 2535756). A mechanical agitator rotating around a horizontal shaft has been mounted in a horizontal reactor of this kind. The drawback in such reactors has been that they cannot be fitted with both partitions and with longitudinal structures increasing the strength of the agitator or with the longitudinal pipes required by the decomposition process. Due to the lack of support structures, it has been necessary to make the agitator shaft extremely strong or to support it with intermediate pillars.

Thus the aim of the invention under condideration is a reactorforthe anaerobic digestion of organic sludge and for the production of methane gas in which there is essentially a horizontal cylindrical vessel containing an agitator rotating around a horizontal axis. In one end of the vessel there is an inlet for the sludge to be digested and in the opposite end an outlet for removing the digested sludge from the vessel, and an outlet pipe in the upper portion of the said vessel for the recovery of the methane gas. In contrast to earlier reactor types, the partitions in the reactor in accordance with the invention under consideration are attached to the agitator, and rotate with it. Thus the partitions are no longer in the way of longitudinal structures in additional to the suport structures.A reactor built in accordance with the invention under consideration differs from earlier reactor types in that there are means at the discharge end of the reactor for returning part of the solid matter settled to the bottom ofthereactorto the input end of the reactor. These means of returning solids are contained entirely within the reactor, in which case there are no problems with seals or risks of leakage. These return means include a pipe which rotates with the agitator and extends from an opening near the periphery of the partition of the settling chamber to the chamber at the inlet end of the vessel. The pipe forms an angle with the agitator shaft. The scraper extending form the opening in the partition of the settling chamber along the bottom thereof pushes the solid matter that has settled to the bottom of the settling chamber into the return pipe.Thus the return means rotate together with the agitator, and the return pipe runs longitudinally through the partitions that rotate together with the agitator, from the settling chamber, at the discharge end of the reactor to a chamber at its inlet end.

The return pipe may be either largely direct, or adapted to follow a spiral-shaped track near the cylindrical jacket of the reactor. The new development with respect to the return pipe is that the pipe is entirely inside the reactor and tghat it has been mounted so that it rotates with the ahitator. In this way the rotating motion of the agitator furthers the advance of the substance to be returned inside the pipe, because the pipe has a spiral shape. When a return pipe that is straight or basically straight is used, the intake and discharge openings are displaced with respect to each other as viewed in the direction of the agitator shaft. Moreover, they are advantageously located on nearly opposite sides of the aagitator shaft. Thus the return pipe slants downwards towards the discharge opening, when the intake opening is at or near its highest point.The incline of the pipe facilitates transfer of the substance towards the outlet. The incline of the return pipe can be made the desired size throughout, regardless of the length of the return pipe, by using a spiral shape for the return pipe. Furthermore, the rotating motion of the agitator facilitates movement of the substance being returned in the return pipe towards its discharge opening. This of course means that the direction of the spiral is the same as the direction of the agitator's rotation, as seen from the input end of the reactor.

In reactors based on the invention, at least the first partition can be moved along the agitator shaft. Thus the size of the first and second sections can be varied according to need. The inlet for the sludge to be digested opens into the first section, and the discharge opening of the return pipe has been advantageously located in the second section so that the seed culture does not come into direct contact with the fresh sludge.

The first partition is mounted so that it can move along the agitator shaft and has been advantageously attached to rod inside the agitator. This rod extends outside the reactor so that the dividing wall can be moved back and forth along the shaft from outside the reactor.

The function of the scraper is to push the viscous sludge which has settled in the settling chamber into the return pipe. The solid content of this viscous sudge is high. When the input end of the return pipe and scraper, which is an extension of it, rise out of the sludge as the agitator turns, the substance to be returned in the return pipe flows down towards the so-called methane section in the input end of the reactor, where it seeds active methane bacteria mass. The agitator is stopped advantageously in a position in which the scraper and the sludge which has collected in it (this sludge has a high solid content) is above the sludge surface of the reactor. Then the substance to be returned runs out of the scaper into the return pipe and from there to the methane section.

Several vertical lamellae that run basically lengthwise in the reactor are attached advantageously to the end of the reactor at such a height from the bottom of the settling chamber that the scaper is able to move underneath them. The scraper is mounted so that it moves near the lower edge of these vertical lamellae.

Thus it cuts off a slice of the thick sludge cake which settles between the baffles without disrupting settling.

Furthermore, there are several long blades running basically the entire length of the reactor with the exception of the settling chamber in the direction of the reactor axis and located near the reactor jacket. The blades can be attached to the partitions or fitted through them if the partition moves along the shaft. Several blades are mounted on the partitions and these blades move along the innter wall of the vessel as the agitator rotates keeping it clean which facilities heat transfer through the jacket from any indirect source of heat used. In addition, the blades rise above the surface of the sludge in the reactor and efficiently disperse the foam on top of the sludge when they sink beneath the sludge surface again. There are no agitator blades in the settling chamber, and they can be left out of one or more of the other chambers as well. The agitator blades are usually attached to the partitions, but if the partition can be moved along the shaft, the agitator blades are mounted so that they can move through the openings at a corresponding point in the moveable partition.

There can otherwise be openings in the dividing walls from which the sludge can run from one section into the next. These openings are not, however, necessary, for the sludge can also get out from between the dividing walls and the reactor jacket.

The invention is explained in more detail by the enclosed drawings, in which Figure 7 is a cross-section of the recommended implementation of the invention.

Figure 2 is a section along line A-A in Figure 1, Figure 3 is a section along line B-B in Figure 1, and Figure 4 is an end view of the discharge end of the reactor shown in Figure 1.

The cylindrical reactor jacket in the enclosed drawings is marked with reference No. 1. The shaft (2) is mounted centrally inside the horizontal cylindrical jacket (1) and it extends through the end wall of the reactor (3) in the discharge end. It continues outside the reactor where it is attached to the power source (4) that turns the shaft (2). The shaft (2) is mounted on bearings in the end walls (3) and (5), from which the bearing mounted in end wall (3) is sealed.

There is an inlet (6) for sludge in the reactor end wall (5). The inlet (6) opens into the reactor approximately at the middle point of the end wall (5), so that the intake channel completely surrounds the bearing of the shaft (2) in the end wall (5), so that the bearing does not need to be sealed. The outlet (7) for the digested sludge is located in the opposite end of the reactor. The outlet (7) opens into the reactor at a point which is slightly above the seal bearing of shaft (2). As figures 1 and 4 show more exactly, there is a control gate (8) in the outlet (7). This gate can be adjusted vertically and used to regulate the height of the sludge surface (9) in the reactor.

At the discharge end of the reactor there is also an outlet pipe (9) for the digester gas. It is mounted on the highest part of the cylindrical jacket (1) so that its discharge opening is above the sludge surface (9). The reactor also containes four revolving partitions (10) mounted at intervals along shaft (2). All are stationary except for the first partition, taken in the direction of flow. This first partition can be moved with the rod (11) extending from outside the reactor into the agitator shaft (2). The rod (11) is attached to the first partition by means of a slide (12) which moves along the shaft (2).

Furthermore, 3 agitator blades (13) have been attached between the partitions (10) near the cylindrical wall so that the blades (13) are at the corners of an imaginary equilateral triangle. There are no agitaator blades (13) in the last chamber of the reactor, the so-called settling chamber (14). Instead, the blades (13) extend through the first partition (10) that moves along the shaft. They run to the first zone, where their ends are joined with the inside surface of the cylindrical jacket (1) that they keep the surface clean as the agitator rotates. They also effectively disperse the foam formed on the surface of the sludge when they sink beneath the sludge surface (9). Their mixing capacity is excellent.

The partitions (10) also have one or more openings (16), through which the sludge can flow from on chamber into another. Moreover, the sludge can flow from the ring-type opening between the periphery of the partitions (10) and the cylindrical jacket (1). The return pipe is marked with reference number 17. The return pipe (17) extends from the opening (20) near the periphery of the last partition (10). through the intermediate partitions. The pipe (17) follows a spiral path through the intermediate partitions (10) near the cylindrical jacket (1), where the centrifugal force is greatest and where the rotating motion of the agitator is therefore most effective in moving the substance to be returned in the return pipe.

Furthermore, a scaper (18) is attached to the last partition (10) which extends from the partition into the settling chamber (14) very near the inner wall of the cylindrical jacket The scaper (18) has a scqop-like shape; the mouth of the scoop is pointed in the direction of the rotating motion so that in its lower position the scraper (18) picks up the matter settled to the bottom of the settling chamber (14) and moves it along. The intake opening (20) of the return pipe (17) opens to the bottom ofthe scoop-shaped scraper (18) runs out of the opening (20) into the return pipe (17) from there into the second chamber. The agitator is stopped advantageously when the scraper (18) is in the upper position. Thus the matter in itto be returned runs into the return pipe (17) by gravitational force.

Furthermore, there are lamellae (19) attached to the end wall of the reactor (3) in the settling chamber (14).

The lamellae are vertical and run lengthwise with respect to the reactor axis. Their purpose is to reduce any currents in the settling chamber (14) and facilitate settling of the solid matter from the sludge. The lamellae (19) are of such a height from the bottom of the settling chamber (14) that the scraper (18) can move freely between the jacket (1) and the lamellae (19), picking up the solid matter that has settled from between the lamellae.

The sludge to be digested, to which ferrous sulphate may have been added, is either fed batchwise or continuously into the reactor through the inlet (6). The sludge is stirred and heated in the first chamber, which could be called the pre-heating chamber. In the second chamber methane bacteria from the return pipe (17) are added and digestion begins. In the second, third and fourth sections the methane develops. The methane leaves the reactor through the outlet pipe (9) in the upper portion of the reactor and is recovered. In the final section, i.e. the settling chamber (14), the solid matter in the sludge settles to the bottom of the settling chamber(14), and a part of this settled sludge is returned by the scraper (18) and via the return pipe (17) to the second chamber. The digested sludge is finally removed through the outlet (7) in which the height of the sludge surface in the reactor is regulated by the control gate (8). The agitator is rotated either continously, or most effectively at intervals, so that the scraper (18) always stops in the upper position. Then the sludge it has picked up moves into the return pipe (17) by gravitational force.

Claims (12)

1. A reactor for anaerobic digestion of organic sludge and production of methane gas, comprising an essentially horizontal cylindrical vessel, a rotatable agitator mounted on a horizontal shaft in said vessel, an inlet for sludge to be digested at one end of said vessel, and an outlet pipe in the upper portion of said vessel for the recovery of methane gas, partitions extending adjacent to the cylindrical jacket of said vessel and mounted at axial intervals on said horizontal shaft for rotation therewith, said partitions dividing said vessel into chambers, of which the chamber adjacent to said outlet forms a settling chamber, and means for returning solids settled in said settling chamber to another of said chambers, said means comprising a pipe extending from an opening in said partition along the bottom of said settling chamber and mounted for rotation together with said agitator, for forwarding solids settled on said bottom to said return pipe.

2. A reactor as in claim 1, wherein said return pipe has an inlet opening and an outlet opening, said openings being displaced with respect to each other as viewed in the axial direction of said agitator, and preferably being situated on opposite sides of the shaft of said agitator.

3. A reactor as in claim 1, wherein said return pipe extends along an essentially spiral-shaped path in the vicinity of the cylindrical jacket of said vessel.

4. A reactor as in any of claims 1, 2 or 3, wherein at least the partition adjacent to said inlet is movable along said shaft.

5. A reactor as in claim 4, wherein said partition is connected to a slide movable along said shaft and in its turn connected to a rod extending from within said shaft to the outside of said vessel, said rod being slidable back and forth for controlling the position of said partition.

6. A reactor as in any of the preceding claims, wherein a plurality of vertical lamellae extending the longitudinal direction of said vessel are provided in said settling chamber at a height above the bottom thereof sufficient to permit said scraper to pass below said lamellae.

7. A reactor as in any of the preceding claims, wherein said agitator includes a plurality of blades extending adjacent to the jacket of said vessel in the axial direction over substantially the entire length of said vessel with the exception of said settling chamber, said blades being secured to said partitions or, in the case of a movable partition, passing through said partition.

8. A method for the anaerobic digestion of organic sludge and production of methane, comprising introducing sludge to be digested into a closed reactor, agitating said sludge in said reactor and optionally heating it, and removing digested sludge and methane gas from said reactor, and further comprising introducing ferro sulphate as a reductant into said vessel either sepaarately or together with said sludge to be digested.

9. A method as in claim 8, comprising causing said sludge to settle in the outlet end of said reactor, and returning a portion of the settled solids within said reactorto the inlet end thereof.

10. A method as in claim 8 or 9, comprising introducing sludge to be digested and ferro sulphate into a reactor as in any of claim 1 to 7.

11. A method for the anaerobic digestion of organic sludge and production of methane substantially as described herein.

12. A reactor for anaerobic digestion of organic sludge and production of methane substantially as described herein with reference to or as illustrated in the accompanying drawings.